Mixtures that contain acetonitrile and other compounds considered as impurities are commonly denominated "crude acetonitrile" and they are originated in the production process of acrylonitrile.
Due to both economic and technical factors, a certain time ago the recovery of acetonitrile from mixtures containing the same, was not considered feasible. However, the present changes in these factors at world level have given rise to the need to carry out various investigations for the development of processes to obtain and purify acetonitrile.
The processes for purifying acetonitrile have as their object to eliminate mainly: acrylonitrile, hydrogen cyanide and water, which are generally the impurities that accompany the acetonitrile. Nevertheless, other impurities may also be present such as 3,3'iminodipropionitrile and epoxides, in insignificant amounts. Prior to the present invention, the methods used have included contacting the crude acetonitrile with chemicals and effecting thereafter operations to separate purified acetonitrile from the remaining compounds.
For examle, in the U.S. Pat. No. 3,322,814, the purification of crude acetonitrile is described, putting the same in contact with KMnO.sub.4, and performing later on a filtration to obtain the purified acetonitrile.
In the U.S. Pat. No. 3,328,458 a process is claimed for purifying a mixture of acetonitrile, with H.sub.2 O, and HCN, which process comprises stirring the crude acetonitrile and adding thereto a certain amount of CaCl.sub.2 and copper acetate, continuing the stirring for 3 hours at a temperature of 25.degree. C., allowing the mixture to stand so as to be able to separate an acetonitrile-rich part and an aqueous part, and then finally adding to the organic layer Ca(OH).sub.2 and HCN, to be able to utilize the said mixture in further industrial applications.
In the French Pat. No. 1,431,919, a process is disclosed for purifying acetonitrile admixed with 4.8% acrylonitrile and 17.3% water, to which mixture 0.023 mols of NaOH is added per kg of mixture and methanol is added in a molar ratio of 4:1 with regards to acrylonitrile. Then the mixture is refluxed for 90 minutes, reducing the acrylonitrile contents up to 0.015%, and thus obtaining the acetonitrile by neutralization of the mixture and by extraction thereof.
In the U.S. Pat. No. 3,451,899, a process for purifying acetonitrile is claimed by means of azeotropic distillation and neutralization, which comprises feeding crude acetonitrile to a distilling column, discharging thereof through the upper part a stream which contains about 100 ppm of HCN, which is treated with 0.25 N, NaOH and 0.25 N, FeSO.sub.4 solutions, and which is contacted thereafter with benzene so as to form an aqueous layer and an organic layer, which, after separation, is washed with water.
The relatively dry acetonitrile is obtained at the bottom of the column and then is distilled in a second column.
There exist other processes for the purification of crude acetonitrile which employ extractive distillation and rectifying operations. However, in these processes problems arise from the formation of azeotropic mixtures such as water-acetonitrile, water-acrylonitrile, benzene-acrylonitrile, which are separated with difficulty.
The previous processes present a series of inconveniences which render them impractical, since they require operations that last for a long time and which are, therefore, expensive.
Said processes generally use chemicals which are difficult to handle, especially on the industrial scale required for purification operations, making their use prohibitively expensive.
Due to the purification processes as mentioned above generally involving separating operations such as decantation and settling, they are batchwise processes which present great technical and economic inconveniences compared to the continuous type of processes.
Most processes known up to this moment only eliminate some of the impurities which are generally present in crude acetonitrile.
In general terms one may say that the present methods of purification result in the formation of undesired by-products, which present problems concerning their elimination or possible reutilization.